The new analysis involved 15 of the 30 top-grossing movies each year from 2013 through 2015, such as "Star Wars: The Force Awakens," "Divergent" and "Mission: Impossible-Rogue Nation."Researchers divided each movie into five-minute segments and trackedwhethera character used a gun to shoot at someone during that interval, regardless of the number of times it occurred.

"It's really a measure of how much throughout the movie there is gun violence," said Dan Romer, lead author of the study and research director of the Annenberg center.

The new report said: "The amount of gun violence in PG-13-rated movies continues to exceed that in movies rated R and does so even more clearly than in 2012."

Read More

Effects of violent media

Most movies, especially PG-13 ones, show a sanitized version of violence where people just fall down, according to Douglas A. Gentile, professor of psychology at Iowa State University.

"People don't scream in pain for hours, and we don't see the reaction of their families dealing with the aftermath of it and all these are things that are real about violence," he said.

Gentile compared the portrayal of violence in Japanese media with that in the United States. Japanese media tend to show far more blood and spend a lot of camera time on the pain and suffering of the victim, which helps reduce the aggression effect, he said.

"You start feeling sympathy for the victim, and you become less likely to be aggressive because you start seeing just how horrible it really is," he said.

When asked about how violent a TV show is, people usually base their rating on how much blood there is, instead of "is there an intent to harm," he added.

According to Hill, it is important to differentiate between aggression and violence. He defined the former as "a behavior that's intended to harm another person who does not want to be harmed, either psychologically or physically," and the latter as "aggression whose goal is extreme physical harm, like injury or death."

He explained that while there is an abundance of literature that links watching violent media to aggressive behavior, there are no studies that link violent media to violent behavior.

"There's very little reason to believe that witnessing violence in movies should cause that extreme reaction. Instead what it does is that it changes the way we think; we start expecting aggression as more acceptable," Gentile said.

Who assigns the ratings?

There isn't a specific quantity of violence that determines a film's rating. The MPAA created the PG-13 rating in 1984 after parents complained about disturbing content in PG-rated movies.

Join the conversation

See the latest news and share your comments with CNN Health on Facebook and Twitter.

"Ratings are assigned by a board of parents who consider factors such as violence, sex, language and drug use, then assign a rating they believe the majority of American parents would give a movie," according to the MPAA.

"The ratings are based on what parents find offensive rather than on what does the science say is healthy for children, and so, I definitely think there's a lot of room for improvement," Gentile said. The ratings should be content-based and should quantify the amount of violence, sexuality or nudity a film contains so that parents are in a position to make an informed decision based on their values, he added.

With PG-13 movies leading at the box office, Romer expressed his concern about the unknown effects that scenes of gun violence have on children.

"We just are concerned about it; we think it's something that the MPAA should be worried about," Romer said.

Hill explained that parents who grew up with similar media might not be sensitive to its effects.

"The role of parents in providing context and in setting limits and in asking questions ahead of time about what we want our children to learn about the world is critical," Hill said.

"The media are very powerful teachers, and we must understand that our children are learning lessons very effectively with everything that they watch and do, and so it's up to us to think in a very active way about what those lessons are."